A General theory is given for a spin assembly under a nearly resonant rotating magnetic field of arbitrary strength. According to the relation between the static local field strength σ0 and the characteristic frequency of fluctuation φ0 of the environment two important cases are classified. A) If the fluctuation of the environment dominates (φ0 >> σ0), the equation becomes essentially stochastic in type and we can safely define. relaxation times in Bloch's sense, with the modification that the restoring force is proportial to the deviation not from the canonical distribution under the constant magnetic field, but from the canonical distribution under the total magnetic field in an instantaneously static system of coordinates (Modified Bloch equation holds) B) If, on the other hand, the static local field dominates (σ0 >> φ0), then the transverse life time essentially depends on the strength of the rotating field, in fact it is prolonged under saturation. This leads to a general and reasonable explanation of “saturational narrowing” and the anomalous saturational behaviour of dispersion mode as have been observed by Redfields and other in the case of nuclear resonance in some metals.
The same kind of classification applies to the case where we have two interacting different species of spin, of which a general treatment is given in following paper
(The contents are given at the end of the paper)
